Process for preparing raw materials for reaction layer of gas permeable electrode

ABSTRACT

Disclosed herein are processes for preparing a raw material dispersion and raw material powders for producing reaction layers of a gas permeable electrode. The raw materials are prepared in one aspect of the invention by mixing fine particles containing a surface-active agent, freezing the mixture to a temperature the surface-active agent loses at least part of its function and then thawing out the same. The resulting material exhibits superior catalytic performance because the particles are dispersed uniformly by lack of the function of surfaceactive agent at the time of freezing. 
     The raw materials are prepared in another aspect of the invention by mixing hydrophilic minute particles coated with ion exchange resin, PTFE and hydrophobic fine powders. The resulting material exhibits superior catalytic performance because catalysts contained in the hydrophilic minute particles are neither liberated nor made to flow so that the initial catalytic activities are substantially maintained even after a relatively long period of use.

This is a division of application Ser. No. 034,499, filed Apr. 3, 1987now U.S. Pat. No. 4,816,431.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing raw materialpowders or a raw material dispersion which is employed in themanufacture of reaction layers of a gas permeable electrode for use in afuel cell, a secondary battery, an electrochemical reactor and the like.

A gas permeable electrode generally comprises a hydrophilic reactionlayer and a hydrophobic gas permeable layer attached to each other, anda collecting member attached to the opposite surface of the gaspermeable layer.

The hydrophilic reaction layer of the gas permeable electrode maycomprise hydrophilic carbon blacks, polytetrafluoroethylene (hereinafterreferred to as PTFE) and hydrophobic carbon blacks, and containssupported catalysts if necessary. In order to conventionally prepare araw material dispersion for the reaction layer, the mixture ofhydrophobic (or hydrophilic) carbon blacks, PTFE, water and asurface-active agent is filtered to form a cake, which is then thermallydried and pulverised.

In such a process for preparing the raw material powders, uniform andfine particles can not be obtained because the dried cake ismechanically pulverised. Even if the thus obtained powders are mixedwith hydrophilic (or hydrophobic) carbon blacks or hydrophilic carbonblacks supporting catalysts and PTFE to form the reaction layer by meansof calcination, hydrophilic portions and hydrophobic portions may not beuniformly and finely dispersed, to lower the catalytic performancebecause the raw material powders are non-uniform and bulky.

Further, in the gas permeable electrode which has been prepared from theabove mixed powders containing the catalysts, the catalysts supported onthe hydrophilic carbon blacks may be liberated to aggregate, or thecatalysts among the PTFE fine powders and the hydrophobic carbon blacksmay flow to aggregate in a relatively short period of use. Consequently,the dispersion density of the catalysts comes to be less uniform and theclusters of the catalysts become more bulky. When an electrolytepenetrates and gas disperses and permeates, they are in contact withonly the surface of the aggregated catalysts and not in contact with theinternal catalysts so that less catalysts can act effectively.Accordingly, the quantity of current flow per unit area is so low thatthe effectiveness is quite unsatisfactory.

The degradation of the catalysts due to the liberation and the flowingis rapid, and the life of the gas permeable electrode may be shorter.

SUMMARY OF THE INVENTION

The present invention has been devised to overcome such disadvantages.It is an object of the present invention to provide a process forreadily producing a large volume of a dispersion of uniformly and finelydispersed raw materials for a gas permeable electrode.

It is another object of the invention to provide a process for preparingraw material powders for producing a gas permeable electrode free fromcatalysts' liberation from hydrophilic carbon blacks or catalysts'flowing from the portions among PTFE fine powders and hydrophobic carbonblacks.

The process of the invention for preparing a dispersion of raw materialsfor reaction layers of a gas permeable electrode comprises mixinghydrophobic (or hydrophilic) carbon blacks, PTFE, water andsurface-active agent, freezing this mixture to the temperature at whichthe surface-active agent loses at least part of its function and thenthawing out the mixture.

On the other hand, the process for preparing raw material powders forreaction layers of a gas permeable electrode comprises mixing fineparticles comprising hydrophilic minute particles coated with an ionexchange resin, PTFE fine powders and the hydrophobic fine powders.

DETAILED DESCRIPTION OF THE INVENTION

The initial mixture of the invention for preparing the raw materialdispersion may comprise hydrophilic carbon blacks or hydrophobic carbonblacks, PTFE powders, water and a surface-active agent, and should notcontain both of the carbon blacks.

The surface-active agent employed in the invention is optionallyselected from the conventional non-ionic surface-active agents.

The freezing speed and the freezing temperature of the mixture maydepend on the desired qualities of the resulting product and thesurface-active agent employed. Especially, the particle size of thedispersion can be readily controlled by varying the freezing speed. Theparticle size may come to be larger with the increase of the freezingspeed and vice versa. The freezing temperature should be one at whichthe surface-active agent loses all or almost all of its function. Thelower temperatures may be employed. The freezing temperature ispreferably -5° to about 50° C. The thawing speed of the frozen mixturemay scarcely influence the qualities of the resulting product.

The thus prepared raw material dispersion may then be mixed with eitherof the hydrophilic carbon blacks or the hydrophobic carbon blacks whichhave not been included in the initial mixture, and any other additivesto prepare the reaction layers of a gas permeable electrode. Theelectrode possesses hydrophilic portions and hydrophobic portionsuniformly and finely distributed so that a larger contact area betweenan electrolyte and gas may be provided to effectively carry out thereaction to promote the catalytic performance.

The hydrophilic carbon blacks containing the catalysts employed in theinvention may be coated with an ion exchange resin for preventing thecatalysts' liberation and flowing.

In this aspect of the present invention, since the process of theinvention for preparing the dispersion of the raw materials for thereaction layers of the gas permeable electrode is, as described earlier,to freeze the mixture of the raw material powders and then to thaw outthe same, the surface-active agent becomes non-functioning at the timeof freezing so that a number of uniform aggregation clusters of the PTFEfine powders can be formed and frozen incorporating the hydrophobic (orhydrophilic) carbon blacks. The aggregation clusters are then thawedout, as they are by thawing, to provide a uniform and finely dispersedraw material dispersion.

On the other hand, hydrophilic minute particles for preparing rawmaterial powders may be either hydrophilic carbon blacks supportingcatalysts or catalyst powders themselves. The ion exchange resin may beany resin provided that it serves to protect the hydrophilic minuteparticles from the catalysts' liberaton or flowing and to allowelectrolyte and gas permeation.

The coating method of the ion exchange resin may be arbitrarily selectedfrom conventional coating techniques, one example of which is that aliquid ion exchange resin diluted with an alcohol may be mixed withhydrophilic carbon blacks and then the solvent is evaporated to form thehydrophilic carbon blacks coated with the ion exchange resin.

The catalysts employed in the invention are preferably selected from thegroup consisting of precious metals including platinum and their alloys,and base metals and their alloys.

The raw material powders thus obtained may be then press-molded to forma reaction layer, which is further attached to a gas permeable layerseparately prepared to obtain a gas permeable electrode.

In this aspect of the present invention, since the catalysts supportedon the hydrophilic carbon blacks in the gas permeable electrode preparedfrom the above raw material powders are coated with the ion exchangeresin, the catalysts are not liberated from the hydrophilic carbonblacks even after a long period of use. Further, the catalysts coatedwith the ion exchange resin do not flow among the PTFE fine powders andthe hydrophobic carbon blacks. Accordingly, the catalysts of which adispersion rate is high do not aggregate and remain uniform and finelydivided. When an electrolyte penetrates and gas permeates, cations maypermeate the ion exchange resin to be in contact with almost all thecatalysts to carry out an effective reaction. In such a way, the amountof current flow per unit weight of the catalysts can be enlarged, andthe effectiveness can be remarkably elevated. Further, since theliberation and the flowing of the catalysts are prevented, the life ofthe gas permeable electrode can be considerably extended.

Several preferred examples of the invention together with a conventionalexample will be described. However, the present invention is notintended to be limited to these examples.

EXAMPLE 1

Hydrophobic carbon blacks having a mean particle size of 420 A, PTFEpowders having a mean particle size of 0.3 u, water and Triton(trademark) X-100 as a surface-active agent were mixed in the proportionof 7:3:1000:100. The mixture was then chilled from the ambienttemperature to -30° C. at the rate of 20° C./hour, and thereafter thefrozen mixture was thawed out to the ambient temperature to provide araw material dispersion for reaction layers of a gas permeable electrodehaving a particle size of 2 10 u.

On the other hand, hydrophobic carbon blacks having a mean particle sizeof 420 A, PTFE powders having a mean particle size of 0.3 u, water andTriton X-100 were mixed in the proportion of 7:3:1000:100. The mixturewas then filtered in a filtering vessel to prepare a cake. The cake wasthen thermally dried at 280° C. for 3 hours, and thereafter pulverisedto provide raw material powders for reaction layers of a conventionalgas permeable electrode having a particle size of 1 to about 100.

As mentioned, while the particle size of the raw material powders of thelatter conventional example was 1 to about 100 which was larger andnon-uniform and the scattering thereof was high, the particle size ofthe former present example was 2 to about 10 which was smaller andnearly uniform, and the scattering thereof was low.

Thereafter, water and a surface-active agent was added to the rawmaterial dispersion according to the present Example and to the rawmaterial powders according to the conventional example to provide tworespective dispersions. The respective dispersions and hydrophiliccarbon blacks having a mean particle size of 390 A and PTFE dispersionhaving a mean particle size of 0.3 u were mixed in the proportion of 3:7in an aqueous solution, which were dried at 280° C. for 3 hours afterfiltration, and then sintered at 380° C. for 3 seconds to providerespective reaction layers having a thickness of 0.1 mm. Upon the use ofthe respective reaction layers in a gas permeable electrode, thecatalytic performance of the electrode employing the conventional rawmaterial powders was low because the hydrophilic portions and thehydrophobic portions were not distributed uniformly and were not dividedfinely, to provide an insufficient contact area between electrolyte andgas. On the other hand, the catalytic performance of the electrodeemploying the raw material dispersion of the present Example was highbecause the hydrophilic portions and the hydrophobic portions weredistributed uniformly and were finely divided, thereby to provide asufficient contact area between electrolyte and gas to sufficientlycarry out a reaction.

EXAMPLE 2

The same procedure as in the Example 1 was carried out except that thehydrophobic carbon blacks having a mean particle size of 420 A and thehydrophobic carbon blacks having a mean particle size of 390 A werereplaced by the hydrophobic carbon blacks and the hydrophilic carbonblacks both of which have a mean particle size of 420 A, respectively.While the particle size of the resulting raw material dispersion afterthe freezing and the thawing was 2 to about 10, the particle size ofthat without the freezing and the thawing was 1 to about 100.

The catalytic performance of the gas permeable electrode of the presentExample was high for the same reasons as mentioned in Example 1.

EXAMPLE 3

Fine powders having been prepared by coating a 0.05 u of Nafion(tradename of ion exchange resin) onto hydrophilic carbon blacks havinga mean particle size of 420 A supporting platinum powders having a meanparticle size of 30 A; PTFE fine powders having a mean particle size of0.3 u; and hydrophobic carbon black fine powders having a mean particlesize of 420 A were mixed in the proportion of 3:4:3 to provide rawmaterial powders of the present Example.

The raw material powders consisting of the mixed fine powders weredisposed on a base plate at a thickness of 0.45 mm and pressed toprovide a gas permeable electrode containing a reaction layer having athickness of 0.1 mm.

The thus obtained gas permeable electrode was used as an air electrodein a fuel cell of which an electrolyte is 20% sulfuric acid. After thelapse of 1000 hours, the platinum fine powders supported on thehydrophilic carbon blacks were not liberated. As a result of an activereaction which occurred by contacting the cations of the penetratedelectrolyte with most of the platinum fine powders through the ionexchange resin, the quantity of current passed per unit weight of theplatinum powders was about 1.5 times as large as that of a conventionalelectrode, which led to the remarkably elevated efficiency.

The life of the gas permeable electrode was about twice as long as thatof the conventional one because the deterioration due to the liberationand the flowing of the platinum fine powders was prevented.

EXAMPLE 4

Fine powders having been prepared by coating a 0.01 u of Nafion(tradename of ion exchange resin) onto platinum having a mean particlesize of 30 A; PTFE fine powders having a mean particle size of 0.3 u;and hydrophobic carbon black fine powders having a mean particle size of420 A were mixed in the proportion of 4:4:3 to provide raw materialpowders of the present Example.

The mixture was treated and used as an air electrode by the sameprocedure as Example 3. After the lapse of 1000 hours, the aggregationof the platinum fine powders which remained in the uniform and finelydivided condition was prevented by the ion exchange resin so that thequantity of current passed per unit weight of the platinum powders wasabout 1.5 times as large as that of a conventional electrode, and thelife of the gas permeable electrode was about twice as long as that ofthe conventional one.

What is claimed is:
 1. A process for preparing raw material powders fora reaction layer of a gas-permeable electrode comprising mixinghydrophilic minute particles coated with ion exchange resin,polytetrafluoroethylene powder and hydrophobic carbon black powder. 2.The process as claimed in claim 1, wherein the hydrophilic minuteparticles comprise hydrophilic carbon blacks supporting catalysts. 3.The process as claimed in claim 1, wherein the hydrophilic minuteparticles comprise catalysts themselves.
 4. A process for preparing rawmaterial powders for a reaction layer of a gas permeable electrode whichcomprises mixing hydrophilic carbon black particles supporting catalystsand coated with an ion exchange resin, with polytetrafluoroethylenepowder and hydrophobic carbon black powder, each of said ingredientshaving a mean particle size less than a micron.